Why all the PTTs?

It seems that many North American institutions are moving more and more to the use of low molecular weight heparin… 
…(mostly Lovenox in the US) and pentasaccharide (Fondaparinux, Arixtra) in place of standard unfractionated heparin. Yet, the lab volume of partial thromboplastin time (PTT, APTT) tests seems to be holding steady. What are all those PTTs for if we are no longer monitoring heparin? And, shouldn’t the volume of anti-Xa heparin assays be increasing? Geo.